Exploration of Helicon Plasmas for Wakefield Accelerators at the Madison AWAKE Prototype

TL;DR

The paper introduces the Madison AWAKE Prototype (MAP) as a versatile platform to develop high-density, uniform helicon plasmas for beam-driven wakefield accelerators. Through a modular, highly instrumented apparatus—including dual helicon antennas, a 50 mT uniform field, a Faraday screen, and advanced diagnostics (interferometry, LIF, and spectroscopy)—the study elucidates how RF power deposition, magnetic field, and flow dynamics shape plasma density and homogeneity. Key findings show that plasma directionality is linked to right-handed helicon modes and can be reversed by switching field or helicity, while antenna length can be engineered to optimize power coupling for target densities; 2D source-rate mappings reveal wall recycling as a dominant fueling mechanism, informing design choices to improve axial uniformity. Collectively, MAP demonstrates techniques to control density, uniformity, and plasma length, providing actionable insights for scalable helicon plasmas in wakefield accelerators and outlining upgrades toward mid-$10^{21}$ m^{-3}$ densities with triple-antenna configurations and terahertz diagnostics.

Abstract

Plasma wakefield accelerators have the potential to revolutionize particle physics by providing lepton collision energies orders of magnitude beyond current technology. Crucially, these accelerators require a high-density, highly homogeneous, scalable plasma source. The Madison AWAKE Prototype (MAP) is a new plasma development platform that has been built as part of CERN's beam-driven wakefield accelerator project AWAKE. MAP uses a dual helicon antenna setup with up to 20 kW of RF power to create plasmas in the low $10^{20}\,\mathrm{m^{-3}}$ range in a highly uniform magnetic field. The project is supported by a range of diagnostics that allow non-invasive measurements of plasma density, ion and neutral flows, and temperatures, and a 3D finite element model that can calculate helicon wavefield and power deposition patterns. In this paper, we present an in-depth overview of MAP's design and construction principles and main physics results. We show that the plasma discharge direction is set by the combination of antenna helicity and field direction and linked to the well-known preference for right-handed helicon modes. We find that the plasma density depends dramatically on the direction of plasma and neutral flow. A detailed measurement of the ionization source rate distribution reveals that most of the plasma is fueled radially by recycling at the wall, a finding with strong implications for optimizing plasma homogeneity. Lastly, we describe how helicon antennas can be engineered to optimize power coupling for a given target density. Together these findings pave the way toward the practical use of helicon plasmas in wakefield accelerators.

Figures (20)

• Figure 1: CAD model of the Madison AWAKE Prototype. 14 magnets create a uniform 50 mT field inside the borosilicate vacuum vessel. The argon plasma is fueled from the left or right side and sustained by two helicon antennas that can be positioned at 11 different axial positions. The plasma is surrounded by a gapless Faraday screen inside the magnets. Laser-induced fluorescence, heterodyne microwave interferometry, and passive spectroscopy are mounted on motorized 2-axis motion platforms. The experiment uses a modular control interface that allows synchronized and scripted firing, data acquisition, and diagnostic positioning.
• Figure 2: Magnitude of the magnetic field in MAP for 222 A through the inner 12 coils and 333 A through the 2 end coils calculated with a COMSOL finite element model. The field is identical at the core and edge of the vacuum vessel and highly homogenous in the central section.
• Figure 3: CAD model of the jumper box used to set the magnetic field direction. Aluminum jumpers on the top and bottom are used to direct current entering from the power supply on the right towards the magnets connected on the left. The magnetic field direction can be set by either crossing the jumpers as shown or arranging them in parallel. The lower jumper is accessed through the black bottom panel.
• Figure 4: Close-up of the Faraday screen and RF chain for a dual antenna setup, with some magnets and structural supports hidden for better visibility. RF power is fed through RG218/U cables into the bottom left side of each matching network, as shown in \ref{['fig:MAPCAD']}. A rigid copper and PTFE coaxial transmission line passes from each matching network up through openings in the Faraday screen and supports the half-helical antennas. A tubular copper mesh between the Faraday screen and matching networks catches any RF emissions leaking through the screen feedthrough. As shown here, the Faraday screen panels can be removed individually to allow access to the antennas. The bottom of the screen has multiple openings for antenna transmission lines. These feed-throughs are covered when not in use but shown open here for better visibility. Antennas have a 1 cm air gap to the vacuum vessel to prevent excessive heating while the fan array on the top provides cooling to the vacuum vessel.
• Figure 5: Example of MAP antennas before and after bending. The antenna on the right has been used in the majority of studies in this paper. The end hoops are closed using fasteners and the middle tabs connect directly to the transmission line as shown in \ref{['fig:MAPRFChain']}. We use steel fasteners to allow for greater clamping force and ensure a reliable RF connection between the copper parts.